The present invention relates to a burner, and more particularly to an improved swirl burner of the two-stage combustion type in which generation of NO.sub.x or nitrogen oxides and soot is suppressed.
Recently, it has been requested as a legal obligation to reduce unburned noxious compounds such as nitrogen oxides, carbon monoxide, hydrocarbon, etc. generated by burners from the viewpoint of pollution prevention.
In order to meet the requirements as described above, there have conventionally been proposed various methods to suppress generation of such noxious compounds, especially nitrogen oxides or NO.sub.x, one such method being the so-called two-stage combustion method wherein combustion air to be supplied to the burner is divided into primary combustion air and secondary combustion air. Although the two-stage combustion method as described above is known to be very effective for suppressing the generation of NO.sub.x, it has disadvantages in that, when applied to boilers and low temperature furnaces such as petroleum process heaters, etc., flames tend to be excessively long, with a simultaneous increase of the amount of soot in the exhaust gases generated by incomplete combustion. For eliminating such inconveniences, it is necessary to increase the excess rate of combustion air, but such increase is not desirable from the viewpoint of energy saving.
Therefore, development of a low NO.sub.x burner having short flames with a low excess air requirement and yet, having less NO.sub.x and soot generation has been strongly demanded.
Incidentally, the so-called swirl burner, which has been disclosed, for example, in U.S. Pat. No. 3,922,137 entitled "Apparatus for admixing fuel and combustion air", and U.S. Pat. No. 3,852,020 entitled "Method for admixing combustion air in a burner", wherein the combustion air is supplied into the combustion chamber in a swirling or vortex motion through an outer periphery of a fuel nozzle, is extremely effective in the evaporation of the fuel in the combustion chamber, since the flames are caused to swirl in the combustion chamber by the swirling air flow to draw in the combustion gas at its central portion. The known swirl burner as described above, however, still has some points to be improved in the reduction of the excess rate of the combustion air for energy saving, and simultaneous suppression of the amounts of NO.sub.x and soot to be developed.